easyui datagrid 跨页选择

$(document).ready(function () {
        $("#tt").datagrid({
            url: ‘/Control/ajax.ashx?class=DC.BusinessLogic.App.AppAction&method=GetPrint‘,
            rownumbers: true,
            pagination: true,
            idField:‘productid‘,
            height: $(document).height() - 100,
            width: $(document).width(),
            columns: [[
                 { field: ‘productid‘, title: ‘物料ID‘, width: 100, checkbox: true },
                 { field: ‘productcode‘, title: ‘物料编码‘, width: 100 },
                 { field: ‘productname‘, title: ‘物料名称‘, width: 200 },
                 { field: ‘unitname‘, title: ‘单位名称‘, width: 100 },
                 { field: ‘brandname‘, title: ‘品牌名称‘, width: 100 },
                 { field: ‘spec‘, title: ‘规格‘, width: 100 },
                 { field: ‘differenceno‘, title: ‘助记码‘, width: 100 },
                 { field: ‘scalerrate‘, title: ‘换算率‘, width: 100, hidden: true }
            ]]
        });
    })